The most common type of system for heating the passenger compartments (cabs) of heavy duty vehicles is a system that directs hot engine coolant through a heater core and controls the interior temperature of the compartment by controlling the flow of the coolant. Typically, the temperature control is accomplished by manually moving a lever accessible to an occupant of the compartment. Movement of the lever adjusts a flow control valve to thereby adjust the flow of coolant. There is a non-linear relationship between the rate of flow and the air outlet temperature. This makes it difficult to adjust the temperature by means of a lever whose position varies linearly with the flow rate. To overcome this difficulty and enable the operator of the lever to more easily and accurately adjust the temperature, flow control valves have been developed that modulate the coolant flow in a non-linear way to achieve a linear relationship between the hand-operated lever and the air output temperature. One example of such a valve is the valve disclosed in U.S. Pat. No. 3,727,879, granted Apr. 17, 1973, to P. J. Lange et al. This type of valve has been in use in heavy duty vehicle passenger compartment heating systems for a number of years. FIG. 11 illustrates the non-linear relationship between the control lever position and coolant flow typical of the valve. FIG. 12 illustrates the linear relationship between the lever position and the air outlet temperature. Nearly half of the air temperature control is achieved by controlling the first half gallon per minute of coolant flow.
The valve described in the Lange et al. patent is capable of providing the desired lever position to air output temperature relationship illustrated in FIG. 12 when the coolant temperature, delivery flow rate, and pressure are constant. Unfortunately, the actual operation of heavy duty vehicles produces conditions that depart from the ideal condition of a constant value for each of these three parameters. Coolant temperature, flow rate, and delivery pressure vary as the engine load and engine rpm vary. The thermostat in the engine's cooling system limits the variation in temperature. The flow rate and delivery pressure are a function of the engine rpm and the setting of the heating system control valve. When the valve lever is positioned to control flow in the first half gallon range, changes in engine rpm can result in unacceptable variations in flow rate.
In addition to the difficulty in maintaining the desired temperature control during certain operating conditions, other problems have been encountered in connection with the use of the type of control valve discussed above. When the vehicle's engine is operated at continuously high rpm's, such as during prolonged driving at high speeds, a high pressure differential is placed across the flow control valve if the valve is in its off or a slightly open position. The high pressure differential can make the manual operation of the lever very difficult. The difficulty of operation has been observed, in particular, in connection with heating systems in which the flow control valves are Boden cable operated. This type of operation is very common. The high pressure differential also places severe stress on the valve, and especially its seal and metering components. This stress shortens the operational life of the valve.